R-Fe-B sintered magnet

Abstract

An R—Fe—B base sintered magnet is provided comprising a main phase containing an HR rich phase of (R′,HR)2(Fe,(Co))14B wherein R′ is an element selected from yttrium and rare earth elements exclusive of Dy, Tb and Ho, and essentially contains Nd, and HR is an element selected from Dy, Tb and Ho, and a grain boundary phase containing a (R′,HR)—Fe(Co)-M1 phase in the form of an amorphous phase and/or nanocrystalline phase, the (R′,HR)—Fe(Co)-M1 phase consisting essentially of 25-35 at % of (R′,HR), 2-8 at % of M1 which is at least one element selected from Si, Al, Mn, Ni, Cu, Zn, Ga, Ge, Pd, Ag, Cd, In, Sn, Sb, Pt, Au, Hg, Pb, and Bi, up to 8 at % of Co, and the balance of Fe. The HR rich phase has a higher HR content than the HR content of the main phase at its center. The magnet produces a high coercivity despite a low content of Dy, Tb and Ho.

The invention claimed is: 
     
       1. An R—Fe—B base sintered magnet of a composition consisting essentially of 12 to 17 at % of R which is at least one element selected from yttrium and rare earth elements and essentially contains Nd, 0.1 to 3 at % of M 1  which is at least one element selected from the group consisting of Si, Al, Mn, Ni, Cu, Zn, Ga, Ge, Pd, Ag, Cd, In, Sn, Sb, Pt, Au, Hg, Pb, and Bi, 0.05 to 0.5 at % of M 2  which is at least one element selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W, 4.8+2×m to 5.9+2×m at % of boron wherein m is at % of M 2 , up to 10 at % of Co, up to 0.5 at % of carbon, up to 1.5 at % of oxygen, up to 0.5 at % of nitrogen, and the balance of Fe, and containing an intermetallic compound R 2 (Fe,(Co)) 14 B as a main phase, wherein
 the magnet contains the main phase and a grain boundary phase between grains of the main phase, the grain boundary phase containing a (R′,HR)—Fe(Co)-M 1  phase in the form of an amorphous phase and/or nanocrystalline phase having a grain size of up to 10 nm, the (R′,HR)—Fe(Co)-M 1  phase consisting essentially of 25 to 35 at % of (R′,HR), 2 to 8 at % of M 1 , up to 8 at % of Co, and the balance of Fe wherein R′ is at least one element selected from yttrium and rare earth elements exclusive of Dy, Tb and Ho, and essentially contains Nd, and HR is at least one element selected from Dy, Tb and Ho, 
 the main phase contains an HR rich phase of (R′,HR) 2 (Fe,(Co)) 14 B at its surface portion, the HR rich phase having a higher HR content than the HR content of the main phase at its center, and 
 the HR rich phase has a thinnest thickness of at least 0.01 μm. 
 
     
     
       2. The sintered magnet of  claim 1  wherein the HR rich phase is non-uniformly formed at the surface portion of the main phase. 
     
     
       3. The sintered magnet of  claim 1  wherein R contains Nd, and at least one element of Pr, La, Ce and Gd, and an atomic ratio of Nd to said at least one element of Pr, La, Ce and Gd is 75/25 to 85/15 in the total of R. 
     
     
       4. The sintered magnet of  claim 1  wherein R contains Nd and Pr, and an atomic ratio of Nd to Pr is 77/23 to 83/17 in the total of Nd and Pr. 
     
     
       5. The sintered magnet of  claim 1  wherein in the HR rich phase, the HR content is at least 20 at % based on the total of R′ and HR. 
     
     
       6. The sintered magnet of  claim 1  wherein in the HR rich phase, the HR content is at least 31 at % based on the total of R′ and HR. 
     
     
       7. The sintered magnet of  claim 1  having a remanence Br of at least 1.2 T at 23° C. 
     
     
       8. The sintered magnet of  claim 1  having a coercivity Hcj of at least 1,274 kA/m at 23° C.